Electron source for mass spectrometers having an elongated filament and a concave focusing electrode with a resistive coating



F. J. KARLE ETAL Oct. 10, 1967 ELECTRON SOURCE FOR MASS SPECTROMETERS HAVING AN ELONGATED FILAMENT AND A CONCAVE FOCUSING ELECTRODE WITH A RESISTIVE COATING 2 Sheets-Sheet 1 Filed Dec. '7, 1964 r N03 v E EKP G w/ew 1m@ Mana aw nf@ Y B Gef. 10, 1967 F, J. KARLE ETA; 3,346,735

ELECTRON SOURCE FOR MASS SPECTROMETERS HAVING AN ELONGATED FILAMENT AND A CONCAVE FOCUSING ELECTRODE WITH A RESISTIVE COATING 2 Sheets-Sheet 2 Filed Dec. 7, 1964 mw MT mms m E W3@ n ./fm mm d if Y nD United seres' Patent o ELEC'IRON SGURCE FOR MASS SPECTROMETERS HAVING AN ELONGATED FILAMENT AND A CNCAVE FOCUSIN G ELECTRGDE WITH A RE- SISTIVE CATING Franklin J. Karle, Natick, Mass., and' Earl D. Ayers, Au-

burn, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Filed Dec. 7, 1964, Ser. No. 416,385 2 Claims. (Cl. 250-41.9)

ABSTRACT F THE DISCLOS This invention relates to electrostatic time-of-ight mass spectrometer apparatus in which a ribbon of electrons is brought to sharp focus along the longitudinal axis of the ion source, ion acceleration and flight tube assembly. The ions produced by `the collision of the electron beam with the sample in the ion source are accelerated by means of substantially uniform accelerating fields through collimating slits in the ion source, ion accelerator, and flight tube and impinge on a detector which usually is an electron multiplier device.

The electron beam source of the apparatus has incorporated therein a focusing electrode in the form of a slotted tube which has means coupled thereto for maintaining a voltage drop along the tube which corresponds to the voltage drop along the filament of the electron beam source.

This invention relates to an improved mass analyzer and particularly to an improved electron source for an electrostatic timeofflight mass spectrometer.

Time-of-liight mass spectrometers of the prior art types have suffered from one or more of the following problems: they have been very expensive; have been bulky and not especially adaptable for quick changes in the type of analytical work in which they were used; had less resolution than was desirable for many proposed uses; were less sensitive than was desired, or took too much down time whenever repairs or modification were made in connection with the instrument.

Accordingly, a principal object of the present invention is to provide an improved time-of-flight mass spectrometer.

Another object of this invention is to provide a time-ofliight mass spectrometer which has an improved electron gun assembly.

An ancillary object of this invention is to provide an improved ion acceleration assembly for use in a time-ofight mass spectrometer.

An additional object of this invention is to provide an improved more compact time-of-ight mass spectrometer.

Yet another additional object of this invention is to provide a time-of-ight mass spectrometer having improved resolution.

A subordinate object of this invention is to provide an improved method of electronically extracting ions from an ion source.

In accordance with this invention, there is provided electrostatic time-of-flight mass spectrometer apparatus in which a ribbon of electrons is brought to sharp focus along the longitudinal axis of the ion source, ion acceleration and flight tube assembly. The ions produced by the ice collision of the electron beam with the sample in the ion source are accelerated by means of substantially uniform accelerating fields through collimating slits in the ion source, ion accelerator, and flight tube and impinge on a detector which usually is an electron multiplier device.

In further accordance with this invention, the electron beam source has incorporated therein a focusing electrode in the form of a slotted tube which has means coupled thereto for maintaining a voltage drop along the tube which corresponds to the voltage drop along the filament of the electron beam source.

By controlling the voltage drop along the tube to make it correspond to the voltage drop along the length of the filament of the source, better focusing ofthe electron beam may be achieved.

The invention, as well as additional objects and advantages thereof, will best be understood in connection with the accompanying drawings, in which:

FIGURE 1 is a side elevational and block diagram view of mass spectrometry apparatus in accordance with this invention;

FIGURE 1A is an end elevational View of the spectrometer tube shown in FIGURE l;

FIGURE 2 is an end elevational View, partly in section, of an electron source in accordance with this invention;

FIGURE 3 is a side elevational view of the electron source shown in FIGURE 2;

FIGURE 4 is a plan view of the electron source shown in FIGURE 2;

FIGURE 5 is an isometric view of a focusing electrode in accordance with this invention, and

FIGURE 6 is a sectional view taken along the line 6-6 of FIGURE 5.

Referring to FIGURE 1 and FIGURE 1A, there is shown mass spectrometer apparatus 10 in accordance with this invention which comprises an evacuated housing 12 composed of a plurality of sections which as illustrated, an input end section 14 which contains an electron source and ion source, a body section 16 which contains a iight tube, and an output end section 18 which contains a detector. Electrical and vacuum system connections to the various parts of the apparatus are made through headers in the various anges 20, 22, 24, 26 for example.

A vacuum system 28, for example, is coupled to the housing 12 through the flange 24.

A power supply 36 is coupled to electron source and ion source electronic circuitry 30 -through the cable 32.

v A clock generator 34 is coupled to the power supply 36 by means of the cable 38, to the electron source and ion source electronic circuitry 30 through the cable 46 and to the readout device 42, which may be an oscilloscope or char-t recorder, for example, through the cable 44.

The detector is coupled through the header in the'lange 26 and the cable 46 to the readout device 42.

The power supply is coupled, via the cables B, C, and D, to the electron source, the detector (header in flange 26), and the ion sourcev (header in liange 20) respectively.

Referring now especially to FIGURES 2, 3 and 4 it may be seen that the electron gun assembly of this invention, indicated generally by the numeral 50, comprises a blocklike body member 52 having a generally rectangular configuration except near one end 54 which is rounded off to be semi-circular.

The member 50 has a pair of outwardly extending flanges 56, 58 at its lower or non-rounded end 60. The member 50 is coupled to a suitable stem assembly 62 which is part of the fiange (FIGURE 1A) and is adapted to be vacuum sealed to the housing part 14 of the mass spectrometer by means of screws (not shown). The member 50 is coupled to the stem 62 by screws 64, 66. The stem 62, which usually (but not necessarily) is made of metal, has a plurality of pin co-nnector elements 68 extending therefrom on the side of the stern which faces the exterior of the mass spectrometer housing. The pin connector elements 68 are, if the stem is made of an electrically conductive material, insulated therefrom and from one another.

A bore 70 is disposed adjacent to the rounded end 54 of the body member 52, extending completely through the member 52. The bore 70 is perpendicular to the Wall 72 and in axial alignment with the outer surface 74 of the member 52.

The bore 70 has a counter-bore 75, 76 at each end.

An annulus 78, 80 is provided which is made of an electrically insulating material of good thermal conductivity, such as synthetic ruby, for example, and has an outer diameter such that one of the annuli may be press-fitted into each of the counter-bores 75, 76. A slot 82, usually having parallel sides, extends between the inner diameterand outer diameter of each annulus 78 or 80. The width of the slot 82 is equal to or greater than the width of the slot 84 which extends across the top of the rounded end 54 of the body member 62. The slot 82 and the slot 84 are axially aligned with respect to the bore 70.

Referring to FIGURES 5 and 6, as well as to FIG- URES 2 and 3, there is shown a slotted tubular member 86 having an insulating body 144, electrically resistive inner wall surface coating 146 and a generally C-shaped transverse crosssectional configuration. The member 86 is disposed between the annuli 78 and 80, the outer diameter of the tubular member 86 being such with respect to the inner diameter of the annuli that it may be press-fitted between the annuli. The width of the slot 88 in the member 86, which is a beam focusing electrode, is less than or equal to the width of the slot 82 in the annuli 78 or 80.

End clips or caps 140, 142, made of an electrically conductive material such as copper, for example, fit over the ends of the focusing electrode member 86, making electrical contact with the electrically resistive coating 146 on the inner surface of the member 86.

A resistor 148 is coupled across the end caps 140, 142 by leads 156, 158. The resistor may be disposed Within ythe housing 14, connected between and carried by two of the pins 68, for example, or may be disposed outside of the evacuated housing.

A source of potential, illustrated as a battery 154, for example, is coupled across the ends of the resistor 148 in series with a rheostat or variable resistance element 150.

Referring especially to FIGURE 3, as well as to FIG- URES 2 and 4, it may be seen that the rounded end 54 of the body member is fiattened over at least a part of its surface so that, at the flattened part, the thickness of the end Wall is only a few thousandths of an inch (.002 inch is commonly used). The surface 96 of the flattened part ofthe end 54 is substantially parallel with respect to the surface of the end part 60 of the body member 52. The length of the fiattened surface 96 (as measured along the slot 84), is about Vs of the length of the slot 84. The surfaces 98, 100, each beginning at an end of the fiat surface 96, is beveled upwardly at an angle of approximately 45 degrees with respect to an end wise extension of the flat surface 96.

A pair of filament mount support fianges 102, 104 extend outwardly from the wall surfaces 72, 73 of the body member 52 intermediate of the ends 54, 60. The fianges 102, 104 are rigidly coupled to the body member and may, if desired, be an integral part of the block member 52, as shown.

Each of the anges 102, 104 has a bore 106, 108 extending therethrough. The axis of each of the bores 106, 108 is parallel with each other and With the wall surfaces 72, 73. An electrically insulating bushing 110, 112 having an internally threaded bore 114, 116 therein is press-fitted into each of the bores 106, 108 in the flanges 102, 104.

An electron source filament support element 118 or 120 having a threaded end 122 or 124 and a slotted. thinned spring-like end 126 or128 is coupled to each of the threaded bores 114, 116, the slotted thinned ends being so aligned that the bottom of the slots in the ends 126, 128 is below the longitudinal axis of the bore 70 by a distance approximating one half the diameter of the Wire-like electron source (filament) 130.

The filamentary electron source 130 illustrated is a tungsten wire having stop means disposed intermediate of its ends 132,134. While the stop means may be a knot in the wire, it is often easier, from a mechanical construction standpoint, to spot Weld a small metal element to the tungsten wire at an appropriately spaced `distance along the wire. The space between the stop means should be such .that the spring-like ends of the electron source supportl elements holds the wire firmly in tension.

If the filament 130, the slotted tubular member 86, and the slot 88 in the rounded end 54 are properly made and aligned, a single plane should pass midway between the slot 84 and slot 88, and pass all along the length of the filament 130, while the filament 130 is equidistant from any point on the inner resistive surface coating 146 of the focusing electrode 86 (measured perpendicularly).

The ohmic value of the resistor 148 is chosen so that, with the resistance of the coating 146 connected in parallel with it, the rheostat 152 may be adjusted to provide a voltage drop across the length of the member 86 which is the same as the voltage drop along the length of the filament 130.

In operation of the potential on the electron source focusing electrodeY 86 is adjusted to cause the electron beam emanating from the electron source to come as nearly as practical to a line focus in axial and planar alignment with the slot in the ion source assembly as shown in the co-pending application of Roland S. Gohlke and Earl D. Ayers executed concurrently herewith and entitled, Spectrometer Apparatus, Ser. No. 416,384, filed Dec. 7, 1964, noW abandoned.

Because the potential drop along the surface of the resistive coating 146 is adjusted to be the same `as the voltage drop along the filament 130, the electron beam emanating from the source 50 may be brought to a sharper line focus than is possible if the entire surface of the coatingv146 Were at the same potential.

For example, if the voltage drop along the filament 130 is 6 volts and the surface 146 was at a constant potential, then it is obvious that the focusing achieved by the electrode member 86 would be different at different points along the line of the beam. By keeping the potential drop along the surface 146 the same as along the filament, sharper focusing is achieved.

During operation, voltages derived from the electron source and ion source electronic circuitry are repetitively applied to the block-like body member 52 (and thus across the slot 84) of the electron source and to a slotted element (not shown) of the ion source while the remainder of the ion source is held under a constant accelerating field, as shown in the previously mentioned Gohlke and Ayers application.

What is claimed is:

1. Inan electron beam source having an elongated filament, an elongated focusing electrode having a concave surface facing .said filament and disposed in spaced apart relationship along its length, and a slotted electrode, said slot being disposed on the opposite side of said filament from said focusing electrode and spaced further from said 2. A beam source in accordance with claim 1, wherein a separate resistance element is connected in parallel with RALPH G" NILSON Primary Exammer' said resistive coating. A. L. BIRCH, Assistant Examiner. 

1. IN AN ELECTRON BEAM SOURCE HAVING AN ELONGATED FILAMENT, AN ELONGATED FOCUSING ELECTRODE HAVING A CONCAVE SURFACE FACING SAID FILAMENT AND DISPOSED IN SPACED APART RELATIONSHIP ALONG ITS LENGTH, AND A SLOTTED ELECTRODE, SAID SLOT BEING DISPOSED ON THE OPPOSITE SIDE OF SAID FILAMENT FROM SAID FOCUSING ELECTRODE AND SPACED FURTHER FROM SAID FILAMENT THAN SAID FOCUSING ELECTRODE, THE IMPROVEMENT COMPRISING SAID FOCUSING ELECTRODE HAVING AN INSULATING BODY WITH AN ELECTRICALLY RESISTIVE COATING ON SAID CONCAVE SURFACE, AND MEANS TO APPLY AN ELECTRICAL POTENTIAL ACROSS THE LENGTH OF SAID SURFACE. 